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### Points to Remember

• Physics is the study of the behavior of objects in nature on the most fundamental level.

• Understanding basic physical laws and how to apply them is the key to creating dynamic realism in games.

• Velocity is a vector formed by combining speed with a direction.

• Acceleration—also a vector—occurs whenever the velocity changes.

• If you know the acceleration of an object and its current velocity, you can find or project the velocity of that object at any time in the future.

• While in the real world we generally think of speed as being measured in units of distance/time, in Flash we think of speed as being measured by frames. So Flash users usually assume one frame to be one unit of time.

• When looking for the balance between creating smooth-appearing motion and not overtaxing the processors of most computers, 24 frames per second seems to offer the best results.

• The amazingly simple trick for applying “good-enough” gravity to your effects is to come up with a value for gravity and add that value to your y velocity in every frame.

• A frictional force is one that opposes the direction of motion and is caused by the interaction between two materials—in other words, it slows something down.

• Kinetic energy—the energy associated with the momentum of an object—is lost as heat from the friction, and the object slows down.

• “Real” frictional implementation decreases the velocity linearly; “good-enough” friction decreases the velocity by a percentage of the current velocity (nonlinearly).

• In most circumstances, the difference between these is not going to be worth the amount of coding you'd have to put into the ActionScript to arrive at the “correct” implementation.

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